The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: Experiments and models

The low atomic mass elements B, Be, and Li are viewed as sensitive tracers of the involvement of subducted materials in the genesis of island are magmas. In order to better assess the role of dense aqueous fluids in the slab-to-mantle transfer of these elements during subduction, measurements have been made of partition coefficients for B, Be, and Li between aqueous fluid and minerals likely to be present in the basaltic portion of the downgoing slab, namely clinopyroxene and garnet. Experiments at 900 degrees C and 2.0 GPa reveal that the average clinopyroxene-fluid partition coefficient for Be (similar to 2) exceeds that for either Li (similar to 0.2) or B (similar to 0.02) and values are 100X (B,Li) to 1000X (Be) larger than partition coefficients for garnet. Clinopyroxene-fluid partition coefficients were found to vary with the alumina content of run-product clinopyroxenes, but this variation is interpreted to reflect the specific exchange reaction that governs the incorporation of these elements into the pyroxene structure, and not mineral-fluid disequilibrium. The element pairs B-Be, B-Nb, and Li-Yb are considered to be essentially unfractionated during the partial melting process, as evidence by their coherent behaviour in apparently cogenetic lavas and the similarity in their measured mineral-melt partition coefficients. A comparison of clinopyroxene-fluid partition coefficients for these elements with clinopyroxene-silicate melt values reveals that B/Be, B/Nb, and Li/Yb ratios will be significantly fractionated in coexisting aqueous fluid with respect to the residual solid. The elevated ratios of B/Be, B/Nb, and Li/Yb in island are lavas relative to MORE are thus considered to be consistent with an origin by fluid-mediated slab-to-mantle transport. A quantitative model of slab dehydration accompanied by progressive water loss and changes in residual mineral mode reveals that source regions with B/Be and B/Nb appropriate for producing the Izu and Kurile IAB suites can be generated using available estimates for the composition of altered oceanic crust, although B abundances at the high end of published values are required. Because the highest values of B/Be and B/Nb are produced in the mantle wedge at relatively shallow depths, some additional process, such as subduction-induced flow of a hydrated mantle wedge, is required in order to transfer enriched material to depths appropriate for the formation of magmas beneath the volcanic front. Calculations indicate that by the time the slab reaches a depth of 200 km, B/Be and B/Nb in the dehydration residue has been reduced to similar to 5-12% of initial values. Thus, the preferential loss of B during dehydration is viewed as a viable mechanism to prevent the excess B acquired during near-surface alteration of oceanic crust from being cycled into the mantle, thereby maintaining the distinction in B/Be and B/Nb for mantle and crustal reservoirs. Copyright (C) 1998 Elsevier Science Ltd.